Stall control is often used in small wind power units (WPUs) as an alternative to more complex pitch control. Stall control is also being considered for very large WPUs of the future because pitch control can be complex and expensive in very large turbines. In stall control, the speed of the wind turbine is limited to a speed at which the power output is limited by the aerodynamics of the blades regardless of how much wind power is available. A challenge in designing stall control systems is accurately limiting the shaft power of the wind turbine because the power limit is a function of the air density, blade shape, blade pitch and wind turbine speed. In addition, blade soiling reduces performance over time. Generally, these factors cannot be controlled to a high degree of precision, so compromises need to be made.
When a wind turbine is operating in a stall-control regime, depending on the region on the speed-power curve in which the turbine is operating a small change in speed results in a large change in power. Consequently, choosing an optimal speed is difficult. In addition, the repeatability of blade performance and blade pitch is also difficult to maintain. On top of this, the air density at the site of the wind turbine varies with weather conditions and elevation of the turbine. So, for WPUs with fixed speed, the manufacturer or installer needs to change the blade length or pitch to manage the power from site to site and, possibly, from season to season, depending on the location of a particular installation. Generally, what is conventionally done is that a pitch setting is chosen so that the wind turbine provides the optimum power when the air is coldest. This results in the wind turbine providing less-than-optimum power when the weather is warmer. This pitch setting needs to be chosen conservatively so that the wind turbine does not provide too much power so as to overstress the WPU or cause and over-speed situation that would result in the WPU shutting down to protect itself from damage.